The invention concerns an identification device, and especially a code reader, for detecting unidimensional and/or two-dimensional codes with a position-resolving photoreceiver and associated reception optics installed in a housing.
Known identification devices of this type have reception optics that project in its focal plane an image of a code in the object plane. A position-resolving photoreceiver is situated in the focal plane which has several photoreceiver pixels arranged in line or matrix form.
The different contrasts in the code and, consequently, also in the image of the code generate different photocurrents in the individual photoreceiver pixels, from which the contents of the code can be identified with appropriate signal processing.
Such codes are used in a variety of applications, such as for identifying and controlling individual objects on conveyors. The code is not limited to any particular kind of code and includes all types of information media that can be recognized by an image processing device.
Today's sophisticated identification devices are expected to read increasing amounts of information contained in the smallest possible code areas. This means that a good position resolution is demanded of the identification device. Especially high demands are placed on the identification device when the code can be located within a large focus depth range. Sophisticated identification devices must further have a high reading rate, which requires fast reception optics. These two conditions, namely a large focal depth range and a fast reading rate reception optics, require an automatic focusing system.
Known identification devices with an automatic focusing system focus by moving the entire reception optics, or at least subassemblies or individual components thereof, along the optical axis, while the position-resolving photoreceiver remains stationary. For this, the optics is attached to a threaded mount that is typically moved by means of a servomotor. The relatively large mass of the components of fast reading, long-focal-length receiving lenses limits the speed with which they can be moved. Such systems are further subject to mechanical stresses in the region of their bearings and are not well suited for continuous use over long periods of time. Furthermore, this focusing method is generally time-consuming and is therefore not well suited for responding to rapidly changing code distances.
A second group of known identification devices with automatic focusing systems focus by stationarily positioning the receiving optics, while the position-resolving photoreceiver, possibly together with the circuit board carrying it, including additional electronic components, moves in the direction of the optical axis. One such device is disclosed in U.S. Pat. No. 5,245,172. In addition to the mechanical load on such an auto-focusing system for the photoreceiver, including its additional electrical components, the required multistrand electrical connection is subject to continuous movement. Since focusing systems of this type are subject to between about 107 to 109 cycles over their lifetime, interruptions of the electrical connection or failures in the mechanically stressed components are not infrequent.